Narrowband Shear Wave Generation Using Sinusoidally Modulated Acoustic Radiation Force

Feasibility of Phase Velocity Imaging Using Multi Frequency Oscillation-Shear Wave Elastography

OBJECTIVE

To assess viscoelasticity, a pathologically relevant biomarker, shear wave elastography (SWE) generally uses phase velocity (PV) dispersion relationship generated via pulsed acoustic radiation force (ARF) excitation pulse. In this study, a multi-frequency oscillation (MFO)- excitation pulse with higher weight to higher frequencies is proposed to generate PV images via the generation of motion with energy concentrated at the target frequencies in contrast to the broadband frequency motion generated in pulsed SWE (PSWE).

METHODS

The feasibility of MFO-SWE to generate PV images at 100 to 1000 Hz in steps of 100 Hz was investigated by imaging 6 and 70 kPa inclusions with 6.5 and 10.4 mm diameter and ex vivo bovine liver with and without the presence of an aberration layer and chicken muscle ex vivo, and 4T1 mouse breast tumor, in vivo with comparisons to PSWE.

RESULTS

MFO-SWE-derived CNR was statistically higher than PSWE for 6 kPa (both with and without aberration) and 70 kPa (with aberration) inclusions and derived SNR of the liver was statistically higher than PSWE at higher frequency (600-1000 Hz). Quantitatively, at 600-1000 Hz, MFO-SWE improved CNR of inclusions (without and with) aberration on an average by (8.2 and 156)% and of the tumor by 122%, respectively, and improved SNR of the liver (without and with) aberration by (20.2 and 51.5)% and of chicken muscle by 72%, respectively compared to the PSWE.

CONCLUSIONS AND SIGNIFICANCE

These results indicate the advantages of MFO-SWE to improve PV estimation at higher frequencies which could improve viscoelasticity quantification and feature delineation.

Imaging of Single Transducer-Harmonic Motion Imaging-Derived Displacements at Several Oscillation Frequencies Simultaneously

Mapping of mechanical properties, dependent on the frequency of motion, is relevant in diagnosis, monitoring treatment response, or intra-operative surgical resection planning. While shear wave speeds at different frequencies have been described elsewhere, the effect of frequency on the "on-axis" acoustic radiation force (ARF)-induced displacement has not been previously investigated. Instead of generating single transducer-harmonic motion imaging (ST-HMI)-derived peak-to-peak displacement (P2PD) image at a particular frequency, a novel multi-frequency excitation pulse is proposed to generate P2PD images at 100-1000 Hz simultaneously. The performance of the proposed excitation pulse is compared with the ARFI by imaging 16 different inclusions (Young's moduli of 6, 9, 36, 70 kPa and diameters of 1.6, 2.5, 6.5, and 10.4 mm) embedded in an 18 kPa background. Depending on inclusion size and stiffness, the maximum CNR and contrast were achieved at different frequencies and were always higher than ARFI. The frequency, at which maximum CNR and contrast were achieved, increased with stiffness for fixed inclusion's size and decreased with size for fixed stiffness. In vivo feasibility is tested by imaging a 4T1 breast cancer mouse tumor on Day 6, 12, and 19 post-injection of tumor cells. Similar to phantoms, the CNR of ST-HMI images was higher than ARFI and increased with frequency for the tumor on Day 6. Besides, P2PD at 100-1000 Hz indicated that the tumor became stiffer with respect to the neighboring non-cancerous tissue over time. These results indicate the importance of using a multi-frequency excitation pulse to simultaneously generate displacement at multiple frequencies to better delineate inclusions or tumors.

Feasibility of Harmonic Motion Imaging Using a Single Transducer: In Vivo Imaging of Breast Cancer in a Mouse Model and Human Subjects

Harmonic motion imaging (HMI) interrogates the mechanical properties of tissues by simultaneously generating and tracking harmonic oscillation using focused ultrasound and imaging transducers, respectively. Instead of using two transducers, the objective of this work is to develop a single transducer HMI (ST-HMI) to both generate and track harmonic motion at "on-axis" to the force for facilitating data acquisition. In ST-HMI, the amplitude-modulated force was generated by modulating excitation pulse duration and tracking of motion was performed by transmitting tracking pulses interleaved between excitation pulses. The feasibility of ST-HMI was performed by imaging two elastic phantoms with three inclusions (N = 6) and comparing it with acoustic radiation force impulse (ARFI) imaging, in vivo longitudinal monitoring of 4T1, orthotropic breast cancer mice (N = 4), and patients (N = 3) with breast masses in vivo. Six inclusions with Young's moduli of 8, 10, 15, 20, 40, and 60 kPa were embedded in a 5 kPa background. The ST-HMI-derived peak-to-peak displacement (P2PD) successfully detected all inclusions with [Formula: see text] of the linear regression between the P2PD ratio of background to inclusion versus Young's moduli ratio of inclusion to background. The contrasts of 10 and 15 kPa inclusions were higher in ST-HMI than ARFI-derived images. In the mouse study, the median P2PD ratio of tumor to non-cancerous tissues was 3.0, 5.1, 6.1, and 7.7 at 1, 2, 3, and 4 weeks post-injection of the tumor cells, respectively. In the clinical study, ST-HMI detected breast masses including fibroadenoma, pseudo angiomatous stromal hyperplasia, and invasive ductal carcinoma with a P2PD ratio of 1.37, 1.61, and 1.78, respectively. These results indicate that ST-HMI can assess the mechanical properties of tissues via generation and tracking of harmonic motion "on-axis" to the ARF. This study is the first step towards translating ST-HMI in clinics.

Quantifying Dysfunction of the Lumbar Multifidus Muscle After Radiofrequency Neurotomy and Fusion Surgery: A Preliminary Study

The multifidus is an important muscle for the active stabilization of the spine. Unfortunately, clinical procedures such as posterior lumbar fusion (PLF) and radio frequency neurotomy (RFN) cause injury to these muscles affecting their function. However, evaluating multifidus function using traditional biomechanical methods is challenging due to its unique anatomical features. The change in muscle shear modulus during contraction has been corrected to force generation for several skeletal muscles. Therefore, the change in shear modulus can be used to quantify muscle contraction. The objective of this study was to evaluate multifidus dysfunction by comparing changes in shear modulus during muscle contraction in healthy individuals and patients who received RFN and PLF in the lumbar spine. We used our recently developed protocol which consists of measuring changes of multifidus shear modulus at lying prone, sitting up, and sitting up with the arms lifted. In healthy individuals, the median multifidus shear modulus increased progressively from prone, sitting, and sitting with arms raised: 18.55 kPa, 27.14 kPa, and 38.45 kPa, respectively. A moderate increase in shear modulus for these body positions was observed in PLF patients: 9.81 kPa, 17.26 kPa, and 21.85 kPa. In RFN patients, the shear modulus remained relatively constant: 14.44 kPa, 16.57 kPa, and 17.26 kPa. Overall, RFN and PLF caused a reduction in the contraction of multifidus muscles. However, the contraction of multifidus muscle slightly increased during multifidus activation in PLF patients, while it did not change in RFN patients. These preliminary measurements suggest that the proposed protocol using SWE can provide important information about the function of individual spine muscles to guide the design and evaluation of postsurgical rehabilitation protocols.

Measurement of the shear modulus in thin-layered tissues using numerical simulations and shear wave elastography

Measurement of mechanical properties of thin-layered tissues has broad applications in the diagnosis of several pathologies. Ultrasound shear wave elastography (SWE) measures the shear wave speed as a means of estimating the mechanical properties of tissues. However, the wave speed in thin-layered tissues is affected by their thickness and the properties of surrounding tissues. The objective of this study is to introduce a method that combines numerical simulations and SWE measurements to provide a more accurate calculation of shear modulus in layered tissues. In the proposed method, the spatial distribution of the acoustic radiation force (ARF) emitted by the transducer was first computed. The ARF was then used as input for simulating the guided wave propagation in the thin layer with its surroundings. The simulations were repeated for several values of the shear modulus of the layer to obtain the corresponding simulated wave speed. By comparing the measured and simulated wave speeds, a more accurate (corrected) shear modulus can be obtained. The proposed method was validated using experiments in agarose gels. In-vivo SWE measurements were also performed for the fascia of the tibialis anterior (TA) muscle and the aponeurosis of musculotendinous junction (MTJ) in medial gastrocnemius (MG) head in a group of healthy individuals. The simulated and measured wave speed in gel constructs were in good agreement with a maximum error of 7.22%. The average of measured wave speed of fascia and aponeurosis was 3.90 ± 0.16 m/s and 2.33 ± 0.60 m/s, while the corresponding corrected shear modulus was 95.63 ± 17.89 kPa and 6.36 ± 8.98 kPa, respectively. Thickness had a substantial effect on the wave speed in thin-layered tissues with decreasing speed for thinner tissues. The SWE-based simulation method presented in this study has the potential of enhancing clinical assessment for several musculoskeletal conditions involving thin-layered tissues.

Changes in Shear Modulus of the Lumbar Multifidus Muscle during Different Body Positions

Multifidus function is important for active stabilization of the spine, but it can be compromised in patients with chronic low back pain and other spine pathologies. Force production and strength of back muscles are often evaluated using isometric or isokinetic tests, which lack the ability to quantify multifidi contribution independent of the erector spinae and adjacent hip musculature. The objective of this study is to evaluate localized force production capability in multifidus muscle using ultrasound Shear Wave Elastography (SWE) in healthy individuals. Three different body positions were considered: lying prone, sitting up, and sitting up with the right arm lifted. This positions were chosen to progressively increase multifidus contraction and to minimize body motion during measurements. Shear modulus was measured at the superficial and deeper layers of the multifidus. Repeatability and possible sources of error of the shear modulus measurements were analyzed. Multifidus shear modulus (median (IQR)) increased from prone, i.e. 16.15 (6.69) kPa, to sitting up, i.e. 27.28 (15.72) kPa, to sitting up with the right arm lifted position, i.e. 45.02 (25.27) kPa. Multifidi shear modulus in the deeper layer of the multifidi were lower than the superficial layer, suggesting lower muscle contraction. Intraclass correlation coefficients (ICCs) for evaluation of shear modulus by muscle layer were found to be excellent (ICCs=0.76-0.80). Results suggest the proposed protocol could quantify local changes in spinal muscle function in healthy adults; further research in patients with spine pathology is warranted.